An ethylene/vinyl alcohol copolymer (EVOH) is excellent in transparency, gas barrier property against oxygen or the like, solvent resistance, oil resistance, mechanical strength, and the like, and has been molded into a film, a sheet, a bottle, or the like to widely find use in various packaging materials such as a food packaging material, a drug packaging material, an industrial chemical packaging material, and an agricultural chemical packaging material.
The EVOH is typically produced by saponifying an ethylene/vinyl ester copolymer, which is obtained by the copolymerization of an aliphatic vinyl ester such as vinyl acetate and ethylene, in an alcohol solvent in the presence of a catalyst under high-temperature, high-pressure conditions. In recent years, the following process has been known as such production process (see Patent Literature 1). The alcohol solution of the EVOH in a high-temperature, high-pressure state obtained in the saponifying step is brought into contact with water or steam in a vessel so that the alcohol is replaced with the water. The viscosity of the resultant water/alcohol mixed solution of the EVOH stable at normal pressure is adjusted by regulating a ratio between the water and the alcohol or its pressure, and then the solution is melt-kneaded with an extruder. After that, the kneaded product is extruded into a strand shape in a low-temperature coagulating bath mainly formed of water or the like, and is then cut so as to be pelletized. After that, the pellet is washed and dried, and is then subjected to molding into a film, a sheet, a bottle, or the like.
However, the conventional production process for the EVOH has involved the following problem. When the EVOH is turned into a strand by extruding the water/alcohol solution of the EVOH in the coagulating bath, and then the alcohol is washed and removed, the alcohol washed out into the coagulating bath vaporizes in the air to be responsible for the deterioration of a working environment.
In view of the foregoing, the following process has been proposed as a process for solving the problem (see Patent Literatures 2 and 3). Before pelletization for obtaining the final product with an extruder is performed, the water/alcohol mixed solution of the EVOH stable at normal pressure (paste formed of a mixture of the EVOH, and the water and the alcohol) is pelletized once with a kneader or the like while its water content is reduced. After the water-containing EVOH composition pellet thus obtained has been washed and dried, the final product pellet of the EVOH is obtained with the extruder.
However, the production process disclosed in each of Patent Literatures 2 and 3 is disadvantageous in terms of a production cost because an apparatus for producing the water-containing EVOH composition pellet as an intermediate product (hereinafter, such pellet is sometimes referred to as “intermediate pellet”) and an apparatus for drying the intermediate pellet are needed. In addition, the process involves the following drawbacks. It takes time and energy to dehydrate and dry the intermediate pellet. Moreover, the intermediate pellet of the EVOH remaining in the dryer for a long period of time is apt to deteriorate thermally, and there is a high risk that the deteriorated pellet is included in the subsequent product pellet or a subsequent molded article to serve as foreign matter.
Further, the water-containing EVOH composition as an intermediate product typically contains impurities such as the residue of the catalyst to be used at the time of the saponification, a by-product to be produced in saponification, and the alcohol as a solvent. When those impurities are included in large amounts, the EVOH pellet and a molded article using the pellet such as a film, a sheet, or a bottle to be obtained as the final products may suffer from a problem such as coloring. However, the production process for the EVOH (Patent Literatures 2 and 3) involving performing the washing after the production of the intermediate pellet has involved the following problem. As the washing is performed while the object of the washing is in a solid (pellet shape) state, it is difficult to completely remove the impurities from the inner part of the pellet.
In view of the foregoing, the inventors of the present invention have proposed the following processes for producing an ethylene/vinyl alcohol copolymer composition and a pellet thereof (see Patent Literature 4). In the production process for the EVOH composition, before pelletization for obtaining the final product with an extruder is performed, a paste (high-viscosity paste) formed of a mixture of an EVOH, and water and an alcohol is brought into contact with water while being stirred in a stirring vessel so that the composition of the paste is turned into an EVOH composition containing less than 10 parts by weight of the alcohol and 20 to 100 parts by weight of water with respect to 100 parts by weight of the EVOH. Thus, an EVOH composition having a low water content can be obtained without the performance of washing or heat drying after the pelletization.
Citation List
Patent Literature
    [PTL 1] JP 2002-80605 A    [PTL 2] JP 2002-284811 A (corresponding to U.S. Pat. No. 6,838,029 A)    [PTL 3] WO 2004/009313 A1 (corresponding to US 2006108703 A1)    [PTL 4] WO 2009/084509 A1 (corresponding to EP 2228405 A1)